Normally, if target have 100 hp left with 125% counter attack and your sniper kill it with 800 damage, the target will deal 125 counter damage to your sniper not 1000.

This isn’t my video but I found the bug.

In video, at 9:58, Scarlett deal 292 damage to Caedmon with 125% counter attack and Aodhan’s fiend. Caedmon deal 365 counter damage to Scarlett (292 x 125% = 365). This will be possible if Caedmon have exactly 292 hp left and there is no bug.

And at 10:00, Eichelberg deal 379 damage to Cyprian who almost die with 125% counter attack and Aodhan’s fiend. Cyprian deal 473 counter damage to Eichelberg (379 x 125% = 473). I don’t think Cyprian have exactly 379 hp left. And he suppose to deal much less counter damage than that.

Cyprian+20 can go to ~1546 HP depending on emblem path chosen. We don’t know the exact HP b/c budman didn’t show the card but it was a +20 Cyprian. We also don’t know what troop was equipped. If we assume it was the 3* purple reanimated fighters troop, that troop gives +2%HP at lvl1 and +5%HP at lvl 20. If it was a 3* purple reanimated knights troop, that troop gives +5%HP at lvl 1 AND +10% HP at lvl 20.

Therefore a Cyprian+20 with 1546 HP has between 1576 to 1701 HP after troop bonus

Now if we take a look at the total damage inflicted on the enemy Cyprian before Eichelborg fired:
6 dmg from off color tiles
451 from Cillian
370 from c.Colen
257 from Aodhan
89 from burn
25 from fiend
138 from Scarlett

Total damage received = 6+451+370+257+89+25+138 = 1336 damage

1701-1336 = 365 HP
1576-1336 = 240 HP

It’s possible the fiend is somehow getting counted like a minion with HP and is incorporated into the reflected damage.

As you can see Cyprian’s Hp bar from image, it is almost empty. Unless that Cyprian have 2500+ Max Hp. It is impossible for him to have Hp bar like that and have 379 Hp left.